1. Field of the Invention
This invention relates to a photometric device for a camera in which a photometry is conducted on a subject field which is divided into plural photometric regions, thereby calculating an exposure value on the basis of information on brightness of each photometric region.
2. Description of Related Art
There has been conventionally known a camera for performing photometry on a subject field to be photographed which is divided into plural photometric regions to calculate an exposure value on the basis of information on brightness of each photometric region. For example, a camera as disclosed in Japanese laid-open Patent Application No. 1-217428 adopts a so-called "divisional photometry" in which a photographic field (corresponding to a subject field) is sectioned in a matrix form into plural photometric regions. Each of the photometric regions is independently subjected to a photometry to detect information on brightness of each photometric region, that is, a photometric signal (photometric output value) corresponding to each photometric region and obtain a proper exposure value.
A photometric device for the divisional photometry has been known that uses an electric charge accumulation-type of photoelectric conversion device, such as a Charge Coupled Device (CCD) as disclosed in Japanese laid-open Patent Application No. 62-259022, for example. In general, such an electric charge accumulation-type of photoelectric conversion device outputs plural photometric signals, each corresponding to each photometric region, one by one in time series.
Noise components are necessarily contained in the plural photometric signals which are output from the photometric device as described above, and it is believed that these noise components would occur due to the dark current of the photometric device and noises occurring in a photometric circuit. The intensity of the noise components is dependent on temperature of the photometric device, the voltage of a power source, etc., and it is not dependent on the brightness of a subject (in the field). Therefore, for a subject which is brighter than a certain degree (i.e., for a large photometric signal), no problem occurs because the noise component is negligibly small as compared to the photometric signal. On the other hand, when a subject darkens (i.e., a photometric signal becomes small), the percentage of the noise component in the photometric signal is increased, and thus an error in performing photometry is also increased.
Accordingly, when photometric signals are smaller than a predetermined value, the photometric signals in a region are required to be removed from or invalidated for photometric data prior to the calculation of the exposure value, because a photometric error for the region is judged to be large. When such a removal or invalidating operation of the noise-containing photometric signals is carried out, photometric signals for subjects of low brightness in a photographic field can not be obtained. In an extreme case, no effective data is obtained when all of plural photometric signals are smaller than the predetermined value, and thus the photometry must be carried out again to calculate an exposure value. However, such a repetitive photometry causes a time for a photographic operation to be longer, so that the camera loses its high-speed photographing performance. This is unfavorable particularly for a case where a continuous shooting mode for film advance is set in the camera.
Further, when the electric charge accumulation-type of photoelectric conversion element is used as the photometric element, a photometric signal may be small in a case where a time is insufficient for accumulating charge even when a subject has a high brightness, and in this case the above problem is liable to occur.